1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor laser device and a method for inspecting a semiconductor laser bar in the manufacturing process, for the purpose of increasing the accuracy of the length of a resonator.
2. Description of Related Art
In recent years, the use of drives for recording/reproducing DVDs characterized by a large storage capacity has become widespread rapidly in various fields, including video disc players. Moreover, it strongly is desired that the same device can be used for readout of conventionally used CDs, CD-Rs, and CD-RWs as well. For this reason, an optical pickup used for recording/reproducing DVDs and CDs includes as a light source an infrared semiconductor laser with an emission wavelength band of 780 nm for CDs in combination with a red semiconductor laser with an emission wavelength band of 650 nm for DVDs.
As information processing devices such as a personal computer have become more compact, devices for recording/reproducing DVDs and the like also have to be smaller and thinner. To this end, it is indispensable to make an optical pickup smaller and thinner. In order to achieve a smaller and thinner optical pickup, it is effective to simplify a device by reducing the number of optical components. As one means for simplification, a red semiconductor laser and an infrared semiconductor laser may be integrated.
In terms of the reduction in the size of a device and the number of components, a monolithic two-wavelength semiconductor laser recently has been mass-produced, in which a red semiconductor laser and an infrared semiconductor laser are integrated on the same semiconductor substrate. With this configuration, it is possible not only to integrate the semiconductor lasers themselves into one component but also to share optical components such as a collimator lens and a beam splitter between the red semiconductor laser and the infrared semiconductor laser, which contributes to a smaller and thinner device.
Further, to meet the demand for high-speed writing (recording) in recent years, a monolithic two-wavelength laser operable at a high output has been proposed (see JP 2001-345514 A, for example).
Furthermore, as semiconductor lasers have increased output, it is required to make a resonator as an optical waveguide longer so as to improve heat dissipation of a laser chip. For example, it is said that in the case of a semiconductor laser with an emission wavelength band of 650 nm, a resonator length of 1000 μm or more is required to ensure an optical output level of 200 mW or more.
However, the longer the resonator of the semiconductor laser becomes, the more an optical axis shift is affected by a rotation error and the like caused in a process of assembly on a sub-mount. In addition, fluctuations of a coherent length become greater. Thus, in order to make the resonator longer, it is necessary to reduce variations in resonator length itself.
In general, during the manufacture of a semiconductor laser, in order to separate a semiconductor laser wafer having a stripe-shaped light emitting area into bar shapes, scribe lines are formed on a wafer surface in a direction orthogonal to a stripe direction. Then, a load is applied to the wafer along the scribe lines, so that the wafer is cleaved into bar shapes. A method for cleaving a tilted substrate that generally is used for forming a semiconductor laser with an emission wavelength band of 650 nm is disclosed in JP 09(1997)-266347 A, for example.
Cleavage surfaces formed at both ends of each of the laser bars are used as mirrors, and a distance between the mirrors is a resonator length. Thus, variations in resonator length depend on variations occurring in the cleavage process, such as a deviation of a wafer crystal orientation and a deviation of the scribe lines and the like. The resonator length is measured by using a measuring device such as an optical microscope and a SEM (scanning electron microscope) after the cleavage process.
However, according to the above-described conventional method for measuring the resonator length, the measurement is performed for each chip, and thus a total measuring time becomes longer, resulting in more expense and time required for the manufacturing process. Further, according to the measuring method using a microscope, as the resonator becomes longer, the observation field of view has to be broadened, which might result in a considerable measuring error.